Diaryl ethers are useful chemical compounds. For example, diphenyl ether is used as a heat transfer fluid. Likewise, 3-phenoxytoluene is an intermediate in the manufacture of several pyrethrum-type insecticides. Such diaryl ethers have been made by the Ullman reaction. This involves the reaction of a potassium salt of a phenol with a bromo or chlorobenzene in the presence of a copper catalyst.
Another method is Sendersen's reaction in which a phenol or phenol mixture is passed in the vapor phase at about 450.degree. C. over a thorium oxide catalyst.